Printed circuit boards may be used as components in electronic devices. PCBs may be made from multi-layer structures which typically comprise a conductive foil, such as copper foil, and a polymeric resin substrate. The conductive foil forms conductors while the polymeric resin substrate (prepreg) provides structural integrity and forms an insulation between the conductors. Since the conductor and insulator are in intimate contact, adhesion between the two contributes to the performance and reliability of the electronic devices made with them.
Electrodeposited and wrought or rolled copper foils used in the manufacture of printed circuit boards often do not adhere well to the polymeric substrates. The prior practice for achieving adhesion between copper foil and insulating polymeric substrates has been to roughen the copper surface.
Surface roughening has been achieved by several means. The electrodeposited copper foils can be electroformed with a rough surface. On top of this rough surface further roughening is carried out by applying a high surface area treatment. These treatments may be a copper deposited electrolytically in nodular or powder form, or a copper oxide which grows nodular or dendritic, among others. Often times the rolled copper foil has mechanical roughness imparted to it during rolling or by subsequent abrasion. The rolled foils also are conventionally treated with surface area increasing nodular copper or copper oxide treatments.
These surface roughening treatments increase adhesion to the polymers by forming a mechanical interlock with the resin. The mechanical interlock is formed when an adhesive in its liquid state is applied and then cured or when the resin melts and flows prior to cure during lamination. The polymers flow around the roughened surface area treatments to form the mechanical interlock. Since, in some instances, surface roughening treatments detrimentally effect the electrical characteristics of conductive metal foil, their use is disfavored in such instances.
There are several factors contributing to the adhesion measured between the copper foil and the polymeric resin. Some of these are surface area, type of roughness, wettability, chemical bond formation, type of chemical bond, formation of interpenetrating networks, and properties of the adhering materials.
During an adhesion test the interlocked resin and copper often adhere well enough that failure occurs within the resin, a cohesive failure. With some resins the mechanical interlocking of treatment and resin does not result in the desired high adhesion and failure occurs at the interface between resin and copper, an adhesive failure.
Various different prepregs have been used to make the polymeric resin substrate. The most common prepregs are epoxy prepregs, polyimide prepregs and polyester prepregs. When a metal foil is laminated to a prepreg, an adhesion promoting layer containing a silane coupling agent may be used to improve adhesive characteristics.
Epoxy resins have been used for the polymeric resin substrate because of their adhesive strength and thermal properties. In fact, epoxy resins are used frequently because they are inexpensive materials yet possess high reliability for extended periods of time. Many epoxy prepregs on the market are made using amine curing agents, such as dicyandiamide, as a crosslinking agent. However, a number of problems are associated with the use of amine curing agents including environmental, safety and handling concerns. Substantial amounts of certain volatile organic compounds (VOC's), such as organic solvents including methyl cellosolve, dimethyl formamide and propylene glycol ether, are typically used with amine curing agents. For safety and environmental reasons, it is desirable to minimize or eliminate the use of VOC's.
Recently, new prepregs based on epoxy resin systems have been introduced into the market place. The new epoxy prepregs are advantageous because they are made without amine curing agents and typically without substantial amounts of VOC's or with less hazardous organics. However, during subsequent processing of a laminate made of a conductive metal foil and a new epoxy prepreg, such as during an acid etch process, peel strength of the laminate is substantially reduced, sometimes completely reduced. The undesirable result is delamination, wherein the laminate is unfit for use. This is because adhesion between the conductive metal foil and the new epoxy prepreg is destroyed in an acid environment.
Therefore, it is desirable to provide multi-layer structures in which conductive metal foils are securely bound to the new epoxy prepregs described above, especially during subsequent processing of the multi-layer structures. It is also desirable to increase or at least maintain adhesion between the conductive metal foils and the new epoxy prepregs while simultaneously exhibiting high temperature stability.